1. Field of the Invention
The invention relates to a control apparatus for a power transmission system of a hybrid vehicle. More specifically, the invention relates to a technology for alleviating unfavorable effects, on a smooth motion of the hybrid vehicle, of deterioration of start-up performance of an internal combustion engine, which is caused due to a change in the type of fuel that is supplied to the internal combustion engine.
2. Description of the Related Art
There is a power transmission system for a hybrid vehicle in which drive power output from an internal combustion engine is distributed between a first electric motor and an output member that is connected to drive wheels. When the vehicle travels using the drive power output from the internal combustion engine, a storage unit is charged with electric power produced by the first electric motor. The hybrid vehicle is able to travel using a second electric motor as a drive power source. This type of power transmission system is described in each of Japanese Patent Application Publication No. 2006-321466 (JP-A-2006-321466), Japanese Patent Application Publication No. 2004-204740 (JP-A-2004-204740), and Japanese Patent Application Publication No. 2003-262141 (JP-A-2003-262141). With the control apparatus for the power transmission system according to, for example, JP-A-2006-321466, whether the internal combustion engine should be stopped or driven is determined based on a required torque and a vehicle speed when the vehicle is in motion.
Even in an internal combustion engine that is driven using, for example, petrol (hereinafter referred to as “gasoline”), fuel other than gasoline, for example, biofuel may be used. Accordingly, there has been a demand for development of a vehicle that is able to travel using multiple types of fuel. Supply of multiple types of fuels having different characteristics such as volatilities to the internal combustion engine may affect the ease of igniting the fuel in the internal combustion engine, that is, start-up performance of the internal combustion engine. Ethanol is ignited less easily because it has a lower volatility than that of gasoline. When ethanol blended fuel, which is formed by blending gasoline and ethanol at predetermined blend ratio, is burned in the internal combustion engine, as the ratio of ethanol to the ethanol blended fuel increases, the start-up performance of the internal combustion engine deteriorates more significantly. The start-up performance of the internal combustion engine is especially significant when the temperature of the internal combustion engine is low.
When the start-up performance of the internal combustion engine deteriorates due to a change in the fuel type, the temperature of the fuel, which is compressed and expanded in the internal combustion engine, needs to be promptly increased to a temperature, at which the fuel can be ignited, by, for example, promptly increasing the rotational speed of the internal combustion engine, in order to smoothly start up the internal combustion engine. In this way, the unfavorable effects of deterioration of the start-up performance of the internal combustion engine on a smooth motion of the hybrid vehicle needs to be alleviated. As deterioration of the start-up performance of the internal combustion engine proceeds, the rotational speed of the internal combustion engine at the start-up time needs to be increased. Therefore, the torque produced by the first electric motor and the torque produced by the second electric motor, which are used to increase the rotational speed of the internal combustion engine, need to be increased.
Usually, the rated torque, which is the maximum torque that can be output from an electric motor, decreases as the rotational speed of the electric motor increases. The control apparatus for the power transmission system described in JP-A-2006-321466 is not designed with a possibility that the fuel type may be changed taken into account. Therefore, the control apparatus described in JP-A-2006-321466 does not change the condition for starting up or stopping the internal combustion engine based on the fuel type. Accordingly, the control apparatus does not change the condition for starting up or stopping the internal combustion engine, even if the torque produced by the first electric motor and the torque produced by the second electric motor, which are used to start up the internal combustion engine, need to be increased due to deterioration of the start-up performance of the internal combustion engine. Therefore, for example, when the internal combustion engine is started up while the vehicle speed is high, the rotational speed of the first electric motor and the rotational speed of the second electric motor increase in accordance with the vehicle speed and the rated torque of the first electric motor and the rated torque of the second electric motor are decreased. Accordingly, the internal combustion engine is sometimes not started up smoothly because the torque produced by the first electric motor and the torque produced by the second electric motor, which are used to increase the rotational speed of the internal combustion engine, are insufficient. This may unfavorably affect a smooth motion of the vehicle.